The invention relates to a safety device for limiting the high pressure in the discharge pressure zone of an air-conditioning compressor with a variable piston stroke or displacement volume. The compressor is of the type known as wobble plate piston pump or swash plate piston pump, wherein the angle of the wobble plate or swash plate can be variably controlled by the pressure differential between the high pressure in the discharge pressure zone and the pressure in the piston-drive chamber, or in some cases by the pressure differential between the piston-drive chamber and the suction pressure zone of the air-conditioning compressor. Compressors of this type therefore have a connector channel that connects the outlet chamber in the discharge pressure zone to the pressure zone of the piston-drive chamber. The connector channel can contain a valve or a by-pass throttle which can have a controlling influence on the pressure differential between the discharge pressure zone and the pressure zone of the piston-drive chamber. In addition, there can be a connection between the pressure zone of the piston-drive chamber and the suction pressure zone containing in some cases a further valve or fixed throttle by which the pressure differential between the piston-drive chamber and the suction chamber can be influenced. A typical variable-displacement compressor of the kind that is used, e.g., in air conditioners for motor vehicles has a piston-drive chamber arranged inside a housing and containing the driving elements for pumping the refrigerant medium. A driving shaft is rotatably supported in bearing elements. A portion of the housing contains a cylinder barrel containing a plurality of cylinder bores. Each cylinder bore contains a movable piston. An inclined swash plate or wobble plate (hereinafter also collectively referred to as a tilted plate) is mounted on the driving shaft and arranged so that its tilt angle can be changed while it rotates with the driving shaft. The pistons are constrained by appropriate mechanical elements to follow the movement of the wobble-plate surface. The lengths of the piston strokes in the cylinder bores are determined by the tilt angle of the wobble plate or swash plate.
The angle of the tilted plate is controlled by adjusting the pressure in the piston-drive chamber in relation to the amounts of pressure acting on the pistons in the discharge pressure zone (high pressure zone) and in the suction zone (low pressure zone). In other words, the difference between the amounts of pressure acting on the respective ends of the pistons can be adjusted by changing the pressure in the piston-drive chamber. The adjustment changes the angle of the tilted plate, whereby the displacement volume of the compressor is changed. Furthermore, throttle devices or valve devices are arranged between the different pressure zones, whereby the pressure in the piston drive chamber can be regulated in relation to the discharge pressure and the suction pressure. In some cases, the valve devices are remote-controlled through electromagnetic devices. In addition, compressors of the known state of the art have safety devices to limit the maximum pressure in the discharge zone (high-pressure zone). If the maximum allowable pressure in the discharge zone (i.e., the high-pressure zone) is exceeded, the safety device will release refrigerant to the ambient environment, for example through a burst disc or a pressure-limiting valve. In particular with burst discs, continued operation of the compressor is impossible because the burst disc is destroyed by the excess pressure. With pressure-limiting valves, there is in any case a loss of refrigerant because a part of the refrigerant in the air-conditioning system is released to the ambient environment.